Method and a device for checking the condition of semiconductor valves

ABSTRACT

An electric semiconductor valve (V), for example a valve in a converter for conversion between alternating current and high-voltage direct current, comprises a plurality of semiconductor positions (TS1, TS2, . . . TSN) with mutually series-connected semiconductor devices (T1, T2, . . . TN). With the semiconductor valve energized by an alternating voltage substantially corresponding to the rated voltage thereof, the condition of an optional semiconductor position is checked by generating a test firing signal (FP) and supplying it to the firing channel (L, 1, TCU, 3) of the semiconductor position alone, whereupon an indicating signal (IP) delivered by the indicating channel (TCU, 2, D) of the semiconductor position is studied with respect to time.

TECHNICAL FIELD

The present invention relates to a method for checking the condition ofan optional semiconductor position included in an electric semiconductorvalve, for example a valve in a converter for conversion betweenalternating current and high-voltage direct current, which semiconductorvalve comprises a plurality of semiconductor positions with mutuallyseries-connected semiconductor devices, and to a device for carrying outthe method.

The device comprises signal-generating members for generating andapplying to the firing channel of the semiconductor position alone atest firing signal, a sensing member for timely studying an indicatingsignal delivered by the indicating channel of the checked semiconductorposition, and an evaluation member for determining and indicating theresult of the study.

The term "semiconductor position" in this application means a componentgroup comprising a controllable semiconductor device, such as, forexample, a thyristor or a gate turn-off thyristor (GTO thyristor),resistors and capacitors, arranged in a conventional manner at thesemiconductor device, for voltage protection of the semiconductor deviceand for voltage division with other semiconductor positions included inthe valve, a firing channel for receiving and transmitting firingsignals for the semiconductor device, an indicating unit for generatingan indicating signal when the voltage across the semiconductor device inits forward direction exceeds a predetermined value, and an indicatingchannel for delivering the indicating signal.

The term "semiconductor valve", or just "valve", in this applicationmeans a set of a plurality of semiconductor positions with mutuallyseries-connected semiconductor devices, which during normal operationelectrically function as a unit.

The firing and indicating channels may comprise, in a manner known perse, light guides for non-galvanic signal transmission between differentpotential levels and may then at their end points comprise members forconversion between electric signals and light signals.

The semiconductor device may be electrically fired or directlylight-fired. In the former case, circuits designed in a manner known perse for conversion of a firing signal received in the form of a lightsignal into an electric signal, adapted to be supplied to the gate ofthe semiconductor device, may be associated with the indicating unit. Inthe latter case, the received firing signal may be directly applied tothe semiconductor device, via the firing channel, in the form of a lightsignal. The indicating signal may be of a short type, which means that ashort pulse is delivered when the voltage across the semiconductordevice passes the predetermined value in an increasing direction, or ofa long type, which means that an indicating signal is delivered as longas the voltage exceeds the predetermined value. Long indicating signalsmay consist of a continuous signal or be in the form of a pulse train.

BACKGROUND ART

An electric valve, for example included in a converter for conversionbetween alternating current and high-voltage direct current (HVDCconverter), comprises a usually large number of mutuallyseries-connected semiconductor devices in the form of thyristors. Acontrol system, located at ground potential, for the converter generatesa firing order for the valve and a control system for the valve, alsolocated at ground potential, generates as a result of the receivedfiring order a firing signal for each one of the thyristors included inthe valve. These firing signals are received by a firing channel whichis associated with each thyristor and which transmits the firing signalto an electronic unit associated with each thyristor. The electronicunit, which is at the potential of the thyristor, comprises, in the casewith electrically fired semiconductor devices, circuits for converting afiring signal received as a light signal into an electric firing pulsewhich is applied to the gate of the thyristor, as well as an indicatingunit. Where the thyristots are of a directly light-fired type, theelectronic unit consists of an indicating unit only, which is then notconnected to the gate of the respective thyristor. The indicating signalgenerated by the indicating unit is transmitted via an indicatingchannel to ground potential and is used to ensure, in a manner known perse, that the gate of a thyristor is not supplied with a firing pulseunless its off-state voltage in the forward direction has attained apredetermined value, adapted for a rapid and safe firing, as well as toindicate, by its occurrence, that the respective thyristor is notshort-circuited. For the latter purpose, the indicating signal issupplied to a monitoring device for the valve or the converter, wherebythe occurrence of an indicating signal also means a confirmation thatthe respective indicating channel is in operation. The absence ofindicating signals for a semiconductor position is recorded in themonitoring device or in some storage medium connected thereto. Both thefiring and the indicating channels are usually made as optical fibrelinks and are provided at their end points with members for conversionbetween electric and optical signals.

Further, the electronic unit comprises circuits, arranged in a mannerknown per se, for achieving a voltage-controlled firing of the thyristorin the event that firing in the intended manner by a firing pulseemanating from a firing signal generated by the control system fails tooccur. The voltage-controlled firing is initiated when the off-statevoltage of the thyristor in the forward direction exceeds a certainlevel.

For a general description of the technical background within thetechnical field mentioned, reference is made to Åke Ekstrom: High PowerElectronics HVDC and SVC, EKC--Electric Power Research Center, The RoyalInstitute of Technology, Stockholm, 1990.

With the valve in the off-state, each thyristor takes up part of thevoltage across the valve whereby the voltage division between theindividual thyristors is determined by a voltage divider, comprisingresistors and capacitors, connected in parallel with the thyristors. Thevalve is usually so dimensioned that in the event that one or a fewindividual thyristors, for example because of an internal short circuit,should have no voltage-absorbing ability, the remaining thyristorsduring operation under normal voltage conditions are still able to blockvoltages occurring across the valve. However, a monitoring of theoperation of the thyristors included in the valve is still necessarysuch that faulty units can be replaced during planned maintenance work.According to the prior art, the monitoring is carried out in the mannerdescribed above by observing the indicating signals transmitted to themonitoring device, whereby the absence of an indicating signal,indicating a fault such as a short circuit in the respective thyristoror a fault in the indicating channel, is recorded together with anindication as to which semiconductor position has been found to lackindicating signal.

However, commonly known systems for the above-mentioned monitoring doesnot provide any information as to whether the thyristor has been firedin the intended manner by supplying to the gate of the thyristor afiring pulse emanating from a firing signal generated by the controlsystem, or, in the absence of such a firing pulse, by voltage-controlledfiring. Thus, a fault in the firing channel for a thyristor cannot bediscovered in this way.

SUMMARY OF THE INVENTION

It is an object of the invention to achieve an improved checking of thecondition of an optional semiconductor position and thereby particularlyto make it possible to check whether its firing takes place by a firingpulse emanating from a firing signal generated by the control system, orby voltage-controlled firing.

According to the invention, this is achieved, with the semiconductorvalve energized by an alternating voltage substantially corresponding tothe rated voltage thereof, by gene-rating a test firing signal by meansof a signal-generating member and applying this signal to the firingchannel of the semiconductor position alone at a time when the forwardvoltage across its semiconductor device is positive, whereupon anindicating signal delivered by the indicating channel of thesemiconductor position is studied with respect to time by means of asensing member, and, in dependence on the result of this study, it isdetermined by means of an evaluating member whether the function of thesemiconductor position is correct.

Advantageous improvements of the invention will be clear from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail by describingembodiments with reference to the accompanying drawings, wherein

FIG. 1A shows in the form of a block diagram a known device formonitoring of semiconductor positions in an HVDC converter,

FIGS. 1B-1D show embodiments of certain blocks shown in FIG. 1A,

FIG. 1E shows a schematic equivalence circuit for a thyristor withassociated RC circuits,

FIG. 2 shows in the form of a block diagram an embodiment of theinvention,

FIG. 3 shows variations of thyristor voltage as well as firing pulsesand indicating signals in one embodiment of the invention,

FIG. 4A-4F show variations of thyristor voltage as well as firing pulsesand indicating signals in another embodiment of the invention,

FIG. 5A shows in the form of a block diagram an embodiment of a devicefor checking the condition according to the invention,

FIG. 5B schematically shows a 6-pulse valve bridge included in an HVDCconverter, and

FIG. 5C shows in the form of a block diagram another embodiment of adevice for checking the condition according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description relates to the method as well as to thedevice.

FIG. 1A shows part of a valve V in an HVDC converter. The valvecomprises N mutually series-connected thyristors T1, T2, . . . TN withelectric firing, of which only three are shown in the figure. Associatedwith each thyristor is an electronic unit TCU, which is thus placed at ahigh potential. In the figure, a dash-dotted line marks a dividing linesuch that parts shown to the right of this line are at high potentialand parts located to the left of this line are at ground potential.Associated with each electronic unit is a light-pulse generator L,connected to the respective electronic unit by means of a first lightguide 1, and a receiving detector D, connected to the respectiveelectronic unit by means of a second light guide 2. The light-pulsegenerators can all be activated simultaneously by a firing signal CP,generated by a valve control system VCU, which in turn is activated by afiring order VFO for the valve, generated by a control system (notshown) for the converter. When a light-pulse generator is activated, afiring signal FP'1, FP'2, . . . FP'N is sent therefrom to the electronicunit of the respective thyristor, in which the firing signal isconverted into an electric signal which via a conductor 3 is applied tothe gate of the thyristor to cause the thyristor to carry current. Eachone of the electronic units also comprises an indicating unit of thepreviously mentioned kind and from these indicating units indicatingsignals IP'1, IP'2, . . . IP'N in the form of light signals are emittedvia the light guides 2 to the respective receiving detector D, in whichthe received light signal is converted into a corresponding electricsignal.

FIG. 1B shows an embodiment of a light-pulse generator L, comprising alight-emitting diode 4, FIG. 1C shows an embodiment of a detector Dcomprising a light-sensitive diode 5, and FIG. 1D shows an embodiment ofan indicating unit 6, comprising a resistive voltage divider 61, 62which, by means of conductors 63, 64, is primarily connected over thethyristor and secondarily to the input of a level-sensing member 65, anoscillator 66, which is activated by the level-sensing member, anamplifying member 67, which receives the output signal of the oscillatorand, after adaptation of the signal level, applies its output signal toa light-emitting diode 58. The oscillator 66 generates a pulse trainwhere the pulses may, for example, have a duration of 1 μs and the gapbetween two consecutive pulses have a length of, for example, 5 μs.

Each one of the light-pulse generators L with the associated first lightguide 1, the circuits arranged in the respective electronic unit forconversion of the firing signal into an electric signal, and the guide 3between the outputs of these circuits and the gate of the thyristor thusform a firing channel. Each one of the indicating units including theirconductors 63, 64 for connection across the thyristor as well as theassociated second light guide 2 and detector D form an indicatingchannel.

A firing channel, the associated thyristor with resistors and capacitorsfor voltage protection of the semiconductor device and for voltagedivision with other semiconductor devices as well as the associatedindicating channel form a semiconductor position TS.

In a manner known per se, but not shown here, the output signals fromthe detectors D are used to ensure that firing does not take place untila sufficient voltage across the thyristor has been achieved. Theindicating signals arriving are also each supplied to a respectivebistable flip-flop BV, monitored by a microprocessor common to theseflip-flops. At the beginning of each positive half-period for thealternating voltage applied across the valve, there is thus obtained anindicating signal IP from each indicating unit, except from those whichare connected to a short-circuited thyristor or where there is no signalbecause of a fault in the indicating channel--here is no possibility ofdistinguishing these cases from each other. Nor is it possible todetermine, in those cases where an indicating signal is delivered,whether the firing takes place because of a firing signal emanating fromthe control system of the valve or by voltage-controlled firing.

To accomplish a more complete possibility of carrying out a check of thecondition of an individual semiconductor position, it is foreseenaccording to the invention that an emission of individual firing signalsis made possible in such a way that a special test firing signal can beemitted to a single one of the semiconductor positions whereas the otherones do not receive such a test firing signal.

An example of such a possibility is shown in FIG. 2, where a device 7for checking the condition according to the invention is shown. Thedevice 7 comprises a signal-generating member 71 for generating andapplying to the firing channel of an optional semiconductor positionTS1, TS2, . . . TSN alone a test firing signal FP1, FP2, . . . FPN, asensing member 72 for timely studying an indicating signal IP1, IP2, . .. IPN delivered by the indicating channel of the checked semiconductorposition, and an evaluation member 73 for determining and indicating theresult of the study. Associated with each semiconductor position is asignal-selector gate FPG, the output of which is connected to thelight-pulse generator of the position and one input of which receivesthe firing signal CP, generated by the valve control system vCU, and theother input of which receives one of the individual test firing signalsFP1, FP2, . . . FPN generated by the signal-generating member. The testfiring signals are each transmitted to the signal selector gates via arespective conductor 8. The light-pulse generators can thus receivefiring signals emanating both from the control system of the valve andfrom the device 7 for checking the condition. The indicating signalsIP1, IP2, . . . IPN delivered by the detectors D are each supplied tothe sensing member 72 included in the device 7 via a respectiveconductor 9.

In an embodiment according to the invention, which is suitably carriedout when alternating voltage is applied to the valve, preferablysubstantially corresponding to the rated voltage of the valve, but whenotherwise the valve is not in operation, the various semiconductorpositions are tested one-by-one by sending thereto, by means of thesignal-generating member, a test firing signal from the device 7 via theconductors 8 and the signal selector gates FPG. This is carried out at atime when forward voltage is applied to the thyristors, and preferablyclose to a angle of 90° for the alternating voltage. To reduce thevoltage stresses on the thyristors, the angle is suitably chosen largerthan 90°. During the testing, none of the energized semiconductorpositions receives firing signals CP emanating from the valve controlsystem VCU.

Thereafter, the time of the arrival of the following indicating signalsfor the other semiconductor positions is studied and compared with theindicating signal which arrives from the semiconductor position whichhas received a test firing signal. When the condition is correct, theindicating signal of the tested semiconductor position shall then bedelayed in relation to the other ones. If this is not the case, there isa fault--no firing has taken place by means of the test firing signal.If it is known, for example by a monitoring during operation accordingto the technique described with reference to FIG. 1A, that normalindicating signals arrive from the tested semiconductor position undernormal operating conditions, and that thus the thyristor itself isfunctioning, the conclusion can be drawn that the thyristor in this unitis regularly fired by means of voltage-controlled firing. A fault isthus present in the firing channel of the semiconductor position andshould suitably be corrected.

It is suitable also to carol out a further testing in such a way thatthe power of the light signals for firing under the same circumstancesis reduced, for example by 30-50%. In the same way it is determined if,as it should, a delayed indicating signal is obtained. If this is notthe case, the safety factor for the firing channel is insufficient,either in the electronic unit or in the light-pulse generator L.

FIG. 5A shows in more detail the embodiment of the device for checkingthe condition of a valve V. The checking device comprises ademultiplexor 712 and a multiplexor 721, with a number of outputs andinputs, respectively, which correspond to the number of semiconductorpositions in the valve, whereby each semiconductor position isassociated with an output on the demultiplexor and an input on themultiplexor. Each one of the outputs of the demultiplexor is connected,via a respective conductor 8, to a signal selector gate FPG and each oneof the inputs of the multiplexor receives, via conductors 9, indicatingsignals from the respective semiconductor position, as described withreference to FIG. 2. A microcomputer 74 transmits via a signal bus 75,in a manner known per se, address information relating to asemiconductor position in the valve and influences the demultiplexor andthe multiplexor such that the indicating signal IP for the addressedsemiconductor position appears on the output of the multiplexor and suchthat a signal FPS on the input of the demultiplexor is supplied via itsoutput to the signal selector gate which is connected to the firingchannel of the addressed position. When a test firing signal is to beemitted, the input of a monostable flip-flop 771 is activated by meansof the microcomputer, which flip-flop, when a signal is supplied to theinput thereof, delivers a signal in the form of a short pulse FPS, forexample of the duration 1 μs, which is supplied to the input of thedemultiplexor and is forwarded therefrom as a test firing signal for theaddressed semiconductor position. The signal FPS is also supplied to theS-inputs of a bistable flip-flop 772 and of a bistable flip-flop 773,the inverted Q-outputs of these flip-flops then assuming the value "0".The indicating signals IP1, IP2, . . . IPN from the semiconductorpositions of the valve are supplied to an OR circuit the output of whichis connected to the R-input of the bistable flip-flop 773. When at leastone of the above-mentioned indicating signals is delivered by therespective indicating channel, the output of the OR circuit and hencealso the inverted Q-output of the bistable flip-flop 773 assume thevalue "1".

The inverted Q-output is connected to a monostable flip-flop 774 which,when a signal is supplied to the input thereof, delivers a signal in theform of a short pulse IPP, which is supplied to a first input of an ANDcircuit 775. The indicating signal IP from the addressed semiconductorposition appears on the output of the multiplexor and is supplied to asecond input of the AND circuit. In the event that the indicating signalIP of the addressed semiconductor position is delivered during the timeduring which the first input of the AND circuit receives a "1" signal,the AND circuit delivers a "1" signal to the R-input of the bistableflip-flop 772 and sets the inverted Q-output thereof at the value "1".This signal constitutes a fault signal VCF, which is supplied to themicrocomputer.

In the event that the indicating signal IP of the addressedsemiconductor position in relation to the indicating signals from theother semiconductor positions is delivered delayed by a time exceedingthe length of the pulse IPP, the output of the AND circuit remains "0"and the inverted Q-output of the bistable flip-flop does not deliver anyfault signal.

By a suitable choice of the length of the pulse IPP, it can be concludedfrom the occurrence of the fault signal VCF that firing of thesemiconductor device in the addressed semiconductor position has nottaken place by means of the test firing signal FP but that firing duringnormal operation takes place by voltage-controlled firing. Anadvantageous choice of the length of the pulse IPP is, for example, 1ms.

The fault signal VCF and information about the address of thecorresponding semiconductor position are recorded in a manner known perse in the microcomputer for storage and retrieval of information aboutthe defective semiconductor position.

An explanation why the invention according to this embodiment is able tofunction resides in how resistors and capacitors for voltage protectionof the semiconductor device and for voltage division cooperate with therespective thyristor. It would go too far to go into all the details ofthese very complicated circuits, but to acquaint oneself with theinvention it is sufficient to refer to the schematic FIG. 1E. Parallelto the series-connected thyristors in a valve, a voltage divider isarranged which comprises voltage divider resistors R1 connected inparallel across each thyristor. Further, a series circuit is arrangedparallel to each thyristor, which series circuit comprises a capacitorC2 connected in series with a resistor R2. Typical values of thecomponents included are R1=40 kohms, C2=3 microfarads, and R2=40 ohms.

In FIG. 3 the upper curve shows the voltage UT across a single one ofthe thyristors as a function of time in case no firing takes place. Eachtime the voltage in the forward voltage direction exceeds a thresholdvalue, which may be 30-100 V, an indicating signal is delivered, heredesignated 10.

The lower curve in FIG. 3 shows the voltage across one single thyristorin case this is fired at the time T. Across the thyristor there is thenapplied a voltage which depends on the time of firing and whichimmediately drops to zero by the thyristor becoming conducting at thetime of firing, whereby the capacitor C2 is discharged through thethyristor while at the same time the thyristor short-circuits theresistor R1. At a certain time E the current through the thyristorbecomes so low that the thyristor stops conducting. The voltage acrossthe thyristor and hence across the other components shown in FIG. 1E isthen essentially zero. The voltage then drops, as shown in FIG. 3, andthen again rises with the alternating voltage but lags behind, so tospeak, in relation to the voltage across the other thyristors in thevalve. Therefore, the indicating signal at the time 11 according to FIG.3 will be delayed in relation to the time 10 for the others, which havenot had their capacitors C2 discharged. After a few periods of thealternating voltage, also the capacitor C2 of the tested semiconductorposition will have recovered. The delay will thus remain, which does notprevent testing other semiconductor positions before full recovery hasbeen made, provided that the fact that the indicating signals of newlytested semiconductor positions are not representative is taken intoconsideration. The above explanation is relatively lucid and theintention is not that it should be a limiting factor for the invention,which is based on the observation of the actual delay between indicatingsignals at the time 10 and the time 11, when only the thyristor of onesemiconductor position has been made conducting.

In another embodiment according to the invention, which is suitablycarried out when the valve is in normal operation and the indicatingunits generate indicating signals of a long type, the varioussemiconductor positions are tested one-by-one by sending thereto a testfiring signal from the signal-generating member 71 via the conductors 8and the signal selector gates FPG in the way described with reference tothe preceding embodiment. This is carried out at a time when forwardvoltage is applied to the respective thyristor but before the time whenthe semiconductor position receives a firing signal CP from the valvecontrol system VCU which is in operation. During the testing, only onesemiconductor position at a time receives a test firing signal.

Thereafter, the time when the indicating signal from the testedsemiconductor position disappears is studied and compared with the timefor emission of the test firing signal. In case of correct condition,the indicating signal of the tested semiconductor position shall thendisappear after a delay ΔT_(min) corresponding to the sum of the timelags in the firing and indicating channels as well as the changeovertime of the thyristor from off-state to on-state. If this is not thecase but an additional delay can be observed before the indicatingsignal disappears, the conclusion can be drawn that the thyristor firesin a voltage-controlled manner. A fault is thus present in the firingchannel of the semiconductor position and should be corrected in asuitable way.

The method is illustrated in FIGS. 4A-4F, which for two semiconductorpositions TS1 and TSN in a valve shows the voltages UT1 (FIG. 4A) andUTN (FIG. 4B) across the respective thyristor as a function of time, aswell as the positions in time for the firing signals FP(1) (FIG. 4C) andFP(N) (FIG. 4D) and the indicating signals IP1 (FIG. 4E) and (FIG. 4F)IPN of the respective semiconductor positions. The firing signals FP(1),FP(N) may emanate from the valve control system or from the checkingdevice. Firing signals emanating from the control system of the valvewhich is in normal operation are designated FPCP and the test firingsignals which are emitted from the checking device are designated FP1and FPN, respectively. The figure illustrates the fact that thesemiconductor position TS1 operates correctly whereas the semiconductorposition TSN does not operate correctly. When the respective thyristoris fired in case of positive forward voltage, the voltage across thethyristor breaks down into a value near zero whereby the correspondingindicating signal disappears. The thyristor T1 is fired, when itscondition is not checked according to the invention, by means of firingsignals emanating from the control system of the valve and itsindicating signal IP1 disappears, when a firing signal has been applied,after the above-mentioned delay ΔT_(min). For the thyristor T1 this alsoapplies to the case where the applied firing signal consists of the testfiring signal FP1. The thyristor T2 is fired by voltage-controlledfiring by means of firing signals, which are initiated by theabove-mentioned circuits for voltage-controlled firing which areincluded in the electronic unit and which are later in time than thefiring signals FPCP. The positions with respect to time for thevoltage-controlled firing are designated VCT in the figure. Nor does thetest firing signal FPN achieve firing of the thyristor TN so the forwardvoltage across this thyristor remains until the voltage-controlledfiring occurs. This fact manifests itself by the indicating signal IPNdisappearing only after a time DT>ΔT_(min) after the emission of thetest firing signal FPN.

FIGS. 5B and 5C illustrate an embodiment of the device 7 for checkingthe condition, which is advantageous when the semiconductor valve isincluded in a converter in a bridge connection. FIG. 5B shows aconverter in a three-phase 6-pulse bridge connection comprising sixvalves V1, V2, . . . V6. The bridge is connected, in a manner known perse, between a three-phase a.c. network, in the figure only designated R,S, T, and a d.c. network, in the figure only denoted by two conductors10, 11. In a manner described with reference to FIG. 2, each one of thevalves receives a firing signal (CP(1), CP(2), . . . CP(6) which iscommon to the respective valve and is generated by a valve controlsystem associated with the valve. A firing order VFO is generated in amanner known per se by a control system for the converter such that thevalves are cyclically supplied with a firing order VFO in the sequenceV1, V2, . . . V6, V1 . . . . Each one of the valves comprises Nsemiconductor positions which, as described with reference to FIG. 2,via signal selector gates FPG, can receive the firing signal CP, whichis common to the valve, as well as individual test firing signals FP.Each one of the semiconductor positions delivers an indicating signal IPof a long type via an indicating channel.

FIG. 5C shows in more detail the embodiment of the device for checkingthe condition of the valve VI. The checking device comprises ademultiplexor 712 and a multiplexor 721, with a number of outputs andinputs, respectively, which correspond to the number of semiconductorpositions in the valve, whereby each semiconductor position isassociated with an output on the demultiplexor and an input on themultiplexor. Each one of the outputs of the demultiplexor is connected,via a respective conductor 8, to a signal selector gate FPG and each oneof the inputs of the multiplexor receives, via conductors 9, indicatingsignals from the respective semiconductor position, as described withreference to FIG. 2. A bistable flip-flop 711 is supplied on its S-inputwith the firing signal CP(6), for the valve V6, which lies before thevalve V1 in the commutating sequence, and on its R-input with the firingsignal CP(1) for the valve V1. The signal CP(6) sets the Q-output of thebistable flip-flop, the output signal of which is designated TO, at "1",which activates a counter 74. When the valve V1 takes up off-statevoltage in the forward direction, indicating signals in the form ofpulse trains are received, as described above with reference to FIG. 1D.The counter 74 transmits via a signal bus 75, in a manner known per se,address information relating to a semiconductor position in the valveand influences the demultiplexor and the multiplexor such that theindicating signal IP for the addressed semiconductor position appears onthe output of the multiplexor and such that a signal FPS on the input ofthe demultiplexor is supplied via its output to the signal selector gatewhich is connected to the firing channel of the addressed position. Amonostable flip-flop 722 which, when a signal is supplied to its input,on its output delivers a pulse the pulse length of which exceeds the gapbetween two consecutive pulses in the indicating signal, in this casefor example 10 μs, converts the indicating signal into a continuousindicating signal IPC. The continuous indicating signal ICP is suppliedto a monostable flip-flop 723 which, when a signal is supplied to itsinput, delivers a short pulse FPT, for example of a length of 1 μs, tothe input of an AND gate 713. This takes place when the indicatingsignal IP and hence the continuous signal IPC appear. The second inputof the AND gate is supplied with the signal TO from the Q-output of thebistable flip-flop 711, and since this has already assumed a "1" state,a signal FPS is generated on the output of the AND gate 713 when theindicating signal IP appears, which signal FPS is supplied to the inputof the demultiplexor and is forwarded therefrom as a test firing signalfor the addressed semiconductor position. The signal FPS is alsosupplied to a time-lag circuit 724, the output of which reproduces theinput signal with a delay t1 when a signal is supplied to the inputthereof and a delay t2 when the signal disappears. The output signal TWfrom the time-lag circuit and the output signal from the flip-flop 722are supplied to an AND gate 73, whereby the signal TW can be said toprovide a time window for study of the indicating signal IPC convertedinto continuous state. In case the indicating signal IPC is stillpresent within this time window, the AND gate 73 delivers on its outputa fault signal VCF. By a suitable choice of this time window, that is,of the delay t1 and the delay t2 for the time-lag circuit 724, theconclusion can be drawn from the occurrence of the fault signal VCF thatfiring of the semiconductor device in the addressed semiconductorposition has not taken place by means of the test firing signal FP butthat firing has taken place by voltage-controlled firing. Anadvantageous choice of the times t1 and t2 can, in this example, bet1=20 μs and t2=30 μs.

The fault signal VCF and information about the address of thecorresponding semiconductor position are supplied to a recording deviceM of a kind known per se, for example a memory associated with amicrocomputer, for storage and retrieval of information about thedefective semiconductor position. Recordation of information as to whichsemiconductor position has been found to have a faulty condition iscarried out in a manner known per se, and schematically indicated inFIG. 5C, by supplying address information from the signal bus 75 via anAND gate 731 to the recording device M when the fault signal VCF ispresent.

When the firing signal CP(1) for the tested valve V1, generated by thevalve control system, is supplied to the R-input of the bistable firstflip-flop 711, the output signal TO from the Q-output thereof assumesthe value "0", which deactivates the counter 72 and blocks the first ANDgate 13. Before the next firing of the valve V1, the output signal TO onthe Q-output of the bistable first flip-flop 711 is again set at "1" bythe firing signal CP(6) for the valve V6, whereby the counter 72 stepsone step to influence the demultiplexor and the multiplexor via the bus75 such that the signals FPS on the input of the demultiplexor and IP onthe output of the multiplexor, respectively, are connected to thesemiconductor position in the valve which is next to be checked, forexample a valve directly series-connected to the valve of the precedingcheck. Semiconductor positions, the semiconductor devices of which arepermanently short-circuited or the indicating channels of which aredefective, do not deliver any indicating signal and will not therefore,when addressed by the checking device, be supplied with any test firingsignal.

Checking devices corresponding to the one described with reference toFIG. 5C are arranged for each of the valves, the S-input of the firstflip-flop 711 being supplied with the valve firing signal CP(n-1) andthe R-input of the flip-flop being supplied with the valve firing signalCP(n), where index n refers to the valve which is to be checked andindex n-1 refers to the valve which immediately precedes the formervalve in the commutating sequence. The recording device M can, ofcourse, be common to one or more valves and/or converters andfurthermore be adapted to record, in addition to an addressidentification for the semiconductor position, also an identification ofthe respective valve and/or converter.

The device shown in FIG. 5C can also be used for a valve which is notpart of a bridge connection. In that case, the S-input of the bistablefirst flip-flop 711, instead of being supplied with the firing signalCP(6), as is the case with the bridge connection, is supplied with asignal CALFA for setting the Q-input of the flip-flop at "1" This ismarked in FIG. 5C by CALFA within parenthesis at the S-input of theflip-flop 711. In this case it is advantageous to supply the signalCALFA at an electrical angle which, counting from the rising zerocrossing of the alternating voltage, is smaller than the minimum controlangle of the semiconductor valve during normal operation, for example10°.

The checking devices shown may in applicable parts, wholly or partially,consist of, for example, a microcomputer, programmed according to theprinciples which are clear from the above description, or of hard-wiredcircuits provided for the particular purpose.

As long as the firing via the firing channel is operating, the systemhas a redundancy such that drop-out of a firing channel permits aprotective function with voltage-controlled firing to take over. If thefiring via the firing channel drops out, this redundancy is no longerpresent. If both of the mentioned methods for firing drop out, thethyristor will break down and become permanently short-circuited.

By the invention it is possible to carry out an automatic check of allof the semiconductor positions included in the valve. It is often aquestion of very large plants with a large number of positions. In eachvalve the number of thyristors may be up to 250, and in a 12-pulsesystem the number of units to be tested can thereby amount to the orderof magnitude of 3,000 units. The alternative of carrying out a manualmeasurement check of all the positions, which would otherwise benecessary, is therefore a very extensive operation, which can beessentially eliminated by the invention. Only those positions which havebeen diagnosed according to the invention thus need to be overhauledmanually, usually for replacing the entire electronic unit for asemiconductor position.

The invention has been exemplified by a valve where the actualsemiconductor devices consist of thyristors with electric firing. It isto be foreseen that the development of directly light-fired thyristorsand of gate turn-off thyristors, so-called GTO thyristors, will be suchthat also thyristors of these kinds will be used in correspondingapplications, and it should therefore be pointed out that the inventioncan also be applied to checking of the condition of semiconductorpositions comprising these types of semiconductor devices.

The invention can, of course, be applied also to semiconductor valvesintended for other purposes than conversion between alternating currentand direct current.

We claim:
 1. A method for checking the condition of an optionalsemiconductor position included in an electric semiconductor valve andcomprising a plurality of semiconductor positions with mutuallyseries-connected semiconductor devices, comprising the stepsof:energizing the electric semiconductor valve by an alternating voltagesubstantially corresponding to the rated voltage thereof; generating atest firing signal and supplying said firing signal to a firing channelonly of the semiconductor position at a time when the forward voltageacross the semiconductor device of the semiconductor position ispositive; and checking that an indicating signal, supplied by anindicating channel of the semiconductor position, which immediatelyexceeds the supplied test firing signal, is delayed relative toindicating signals from the other semiconductor positions included inthe electric semiconductor valve, whereby a failing delay indicates anincorrect condition of the checked semiconductor position.
 2. A methodaccording to claim 1, further comprising the step of supplying the testfiring signal at an electrical angle greater than 90 degrees asdetermined from the rising zero crossing of the alternating voltage. 3.A method according to claim 1, further comprising the step of supplyingthe test firing signal with reduced power.
 4. A method for checking thecondition of an optional semiconductor position included in an electricsemiconductor valve, said electric semiconductor valve comprising aplurality of semiconductor positions with mutually series-connectedsemiconductor devices, comprising the steps of:energizing the electricsemiconductor valve by an alternating voltage substantiallycorresponding to the rated voltage thereof; generating a test firingsignal and supplying said test firing signal to the firing channel onlyof the semiconductor position when a firing channel of the semiconductorposition is supplied with firing signals generated by a valve controlsystem for control of the electric semiconductor valve during normaloperation and at a time when the forward voltage across thesemiconductor device of the semiconductor position is positive and whichis earlier than the firing signal supplied by the valve control systemduring the same period of the alternating voltage; and checking that thetime for disappearance of an indicating signal supplied by an indicatingchannel of the semiconductor position is not delayed in relation to thetime of application of the test firing signal, whereby a delay exceedinga given time indicates a faulty condition of the checked semiconductorposition.
 5. A method according to claim 4, comprising the step ofchecking whether the indicating signal is present during a time intervalwhich begins after a given first time after the application of the testfiring signal and ends at a given second time thereafter.
 6. A methodaccording to claim 4, comprising the steps of supplying the test firingsignal at an electrical angle smaller than a minimum control angle ofthe electric semiconductor valve, as determined from the rising zerocrossing of the alternating voltage.
 7. A method according to claim 4,comprising the step of supplying the test firing signal when anindicating signal is delivered by the indicating channel of the checkedsemiconductor position.
 8. A method according to claim 4, wherein theelectric semiconductor valve is included in a converter in a bridgeconnection, and further comprising the step of supplying the test firingsignal to semiconductor positions selected in dependence on firingsignals generated by the vale control system of the converter forcontrol of the electric semiconductor valve during normal operation. 9.A method according to claim 4, further comprising the step of supplyingthe test firing signal only when an indicating signal has been deliveredby the indicating channel of the checked semiconductor position.
 10. Amethod according to claim 4, further comprising the step of supplyingthe test firing signal with reduced power.
 11. A device for checking thecondition of an optional semiconductor position included in an electricsemiconductor valve, said electric semiconductor valve comprising aplurality of semiconductor positions with mutually series-connectedsemiconductor devices, said device comprising:a signal-generating memberwhich generates and supplies to a firing channel only of thesemiconductor position a test firing signal; a sensing member whichdetermines whether an indicating signal delivered by an indicatingchannel of the checked semiconductor position is delayed relative toindicating signals from the other semiconductor positions included inthe electric semiconductor valve; and an evaluating member whichdelivers a fault signal if there is no delay.
 12. A device for checkingthe condition of an optional semiconductor position included in anelectric semiconductor valve, said electric semiconductor valvecomprising a plurality of semiconductor positions with mutuallyseries-connected semiconductor devices, said device comprising:asignal-generating member which generates and supplies to a firingchannel of the semiconductor position alone a test firing signal; asensing member which determines if an indicating signal delivered by anindicating channel of the checked semiconductor position terminateswithin a given time after the application of the test firing pulse; andan evaluating member which delivers a fault signal if the indicatingsignal terminates later than said given time after the application ofthe test firing pulse.
 13. A device according to claim 12, wherein thesensing member comprises an internal-generating member which generates atime interval which begins after a given first time after theapplication of the test firing signal and ends at a given second timethereafter.
 14. A device according to claim 12, wherein thesignal-generating member supplies the test firing signal at anelectrical angle smaller than a minimum control angle of the electricsemiconductor valve, as determined from the rising zero crossing of thealternating voltage.
 15. A device according to claim 12, wherein thesignal-generating member supplies the test firing signal when anindicating signal is delivered by the indicating channel of the checkedsemiconductor position.
 16. A device according to claim 12, wherein theelectric semiconductor valve is included in a converter in a bridgeconnection, further comprising a selector member which selects thechecked semiconductor position in dependence on firing signals generatedby a valve control system of the converter for control of the electricsemiconductor valve during normal operation.
 17. A device according toclaim 12, wherein the signal-generating member supplies the test firingchannel with a test firing signal only when an indicating signal hasbeen delivered by the indicating channel of the checked semiconductorposition.
 18. A device according to claim 12, wherein thesignal-generating member supplies the firing channel of thesemiconductor position with a test firing signal with reduced power.